In fluid flow systems it is often necessary to reduce the fluid pressure, often several hundreds of pounds per square inch, in order to control the fluid flow. This becomes necessary due to either varying input pressure, such as from a steam boiler whose pressure varies, or when the output pressure must be controlled, as when driving the turbine of a propulsion system whose speed varies with flow rate.
For liquids, pressure drops of this magnitude are accompanied by cavitation and intense audible noise. Cavitation occurs in a liquid system when the pressure is reduced below the vapor pressure of the liquid, at which time vapor bubbles form in the liquid. These vapor bubbles subsequently collapse or implode, generating shock waves in the liquid and resulting in noise and severe erosion of the valve materials.
Additionally, audible noise is emitted by any process for reducing fluid pressure, as a portion of the fluid's turbulent energy goes into audible noise. The audible noise can be hazardous to nearby personnel and disfunctional to systems which must operate quietly.
Heretofore, various devices have been used to control or reduce noise in pressure reduction devices, including mufflers, diffusers, attenuation chambers and absorption materials. Directly related prior art includes combination valve/diffusers wherein multiple paths are provided for the fluid flow such that the pressure is dropped in increments within each such path.
Typical of these are Bates U.S. Pat. No. 4,567,915, White U.S. Pat. No. 3,802,536 and Self U.S. Pat. No. 3,954,124 wherein multiple cylinders each containing a plurality of fixed diameter orifices are arranged coaxially with the total flow being controlled by a plug which can be moved down the center of the innermost cylinder such that it covers or uncovers more or fewer orifices as more or less flow is required.
Another typical configuration is represented by Seger U.S. Pat. No. 4,221,037, Kay U.S. Pat. No. 3,917,222 and Barb U.S. Pat. No. 3,894,716 wherein multiple annular disks each containing tortuous paths are stacked with said paths providing radial flow from the center of the stacked disks and total flow being controlled by a plug which is moved down the center of the annulus such that it covers or uncovers more or fewer orifices as more or less flow is required. This alternative is expensive to fabricate and susceptible to plugging. Furthermore, maintaining the disks in alignment is difficult.
The adiabatic pressure drop of a compressible fluid flowing through an orifice with friction is defined by the following equation: ##EQU1## where "dP" is the pressure drop, "P" the pressure, "dA" the change in the orifice area "A", "k" the ratio of specific heats of the fluid, "M" the Mach number, "f" the friction coefficient, "dx" the length of the friction path and "D" the hydraulic diameter of the friction path. Thus there are two ways to effect a pressure drop: (1) Reducing the area "A" of a single orifice by an amount "dA" (where a reduction in "dA" is a negative number), or (2) Increasing the friction length "dx" or the friction coefficient "f".
None of the above referenced patents, as well as other multistage-multipath valve configurations, vary the size of the orifices to control flow; they only vary the total number of the orifices, that is they rely on varying "M" by changing the mass flow rate to vary "dP", with the area "A" of each orifice being fixed. It is desirable in quiet valves to maintain the Mach number "M" below about 0.4 since the audible noise increases as approximately the eight power of the velocity (Sound Pressure Level SPL=f(V.sup.8)). Thus to maintain a low audible noise level, it is desirable to be able to vary the pressure drop dP independently of the Mach number "M", i.e. by varying the area "A". Roberts U.S. Pat. No. 3,645,298 teaches pressure reduction through moving two orifice plates relative to each other (similar to a standard gate valve), but only claims fast valving action, not the above described benefits of separately varying orifice area "A" to achieve audible noise reduction.
A second common treatment of the above described problems, is to effect a pressure drop "dP" by forcing the fluid through a high friction path, i.e. a path with a high coefficient of friction "f" and a long length "dx". The above referenced configurations all embody tortuous or labyrinthine flow paths to effect pressure reductions. For example, Roberts U.S. Pat. No. 3,645,298 places multiple orifice plates in series with each plate of a length ("dx" in the above equation) to achieve the "desired pressure drop" from the friction. (He does not move the plates relative to each other.) For high velocity fluids, such flow creates intense audible noise.
Pressure reduction effected by expansion through a short orifice, i.e. with "dx" small so that there is little friction, is nearly isentropic so that the potential energy of the source is largely converted to kinetic energy which is available to do useful work, as for example the turbine example given above. Pressure reduction through friction, as cited n the above referenced patents, converts a portion of the pressure source's potential energy into thermal energy via friction which is then not available to do useful work. Thus it is desirable to reduce the pressure by methods in addition to friction in order to conserve the potential energy of the fluid source, i.e. make the valve energy efficient. This is contrary to the art as exemplified by Baumann U.S. Pat. No. 3,715,098.
To minimize the length over which the fluid flow transitions from turbulent flow which is noisey to laminar flow which is quiet, it is often desirable to create capillary and orifice shapes with high friction coefficients "f", circlular cross-sections having the smallest friction coefficient with shapes deviating from a circle showing increasing friction coefficients "f" as the ratio of major to minor axis of the cross-sectional shape increasingly deviates from one.
In addition to limiting the absolute levels of radiated acoustic noise, it is often desirable to limit the low frequency portion of the spectrum of the acoustic noise since lower frequencies are transmitted greater distances. The frequency of the acoustic noise from a jet issuing from an orifice is approximately proportional to the fluid velocity and inversely proportional to the orifice diameter, i.e., F=S*V/d where "F" is the frequency, "S" is the Strouhal number and "d" is the orifice diameter. Thus it is desirable to make the orifice diameter "d" small to increase the frequency "F". It is also desirable to vary the diameters "d" of the multiple orifices within a valve component to spread the total acoustic power radiated over a wide frequency range to avoid intense pure tones. It is also desirable to have a cross-sectional shape with major and minor axes which are substantially different such that there are in effect multiple diameters with the result that the resulting audible acoustic is dispersed over a number of frequencies in accordance with the above formula.
It is also desirable to control the fluid flow rapidly in order to minimize the effects from fluctuations in the pressure of the fluid source. This means that a valve must often respond from fully open to fully closed in less than 0.1 second while operating against the large forces, often thousands of pounds in nominal steam flow applications, created by rapidly changing the momentum of the fluid flow (i.e. the direction of fluid flow) during valve closing. It follows that it is desirable to design a valve so that such forces are balanced against one another and the net force required to actuate the valve is low.
Closing valves against high velocity fluids requires large amounts of energy because of the large forces required to change the direction of the fluid and the long distances of travel of valve components against these forces from fully open to fully closed. Therefore, it is also desirable to minimize the travel distance from fully open to fully closed to minimize the work that the actuator must perform. Furthermore, when fast valve response is required, it is desirable to minimize the actuator work in order to minimize the required actuator energy (work per unit time) and therefore actuator size, cost and actuator power consumed. The prior art does not incorporate these features.
Pressure reductions in high velocity fluids can cause the condensation droplets which impinge upon valve surfaces causing severe erosion of the surface leading to premature valve failure. Heretofore, this necessitated valves constructed of special erosion resistant materials. The erosion rate can be approximated by the following equation: EQU log(ER)=4.8*log(Vn)+0.67*log (dia)-16.65-log(NEA)
where "ER" is the rationalized erosion rate, "Vn" the component of the droplet velocity normal to the valve component surface, "dia" is the droplet diameter of the condensed fluid and "NEA" is the material erosion resistance number. The prior art has been to use materials of high "NEA", for example Stellite 6 which has an "NEA" value of 30 versus austenitic stainless steels (common valve materials for low performance valves) which have "NEA" value of approximately 1. Materials with a high "NEA" value are expensive and difficult to fabricate making erosion resistant valves expensive. It is desirable to design a valve to limit the diameter of the droplets "dia" and also to limit the normal component of the velocity "Vn" as an alternative to using high "NEA" materials.
It is also desirable for a fluid control valve to be self-cleaning, i.e. for the orifices to be able to release foreign particles contained in the fluid before valve operation is impaired.